A few months after setting up a new tank and transferring all the livestock from the old one, I got an increasingly bad algae bloom. I read books, scoured the web, and got very frustrated because I was supposedly doing all the right things and still had a furry tank.
I read a lot of posts by people who came through algae blooms, and the general story went something like this: I tried a lot of stuff, and it eventually went away.
I'm not trying to make it sound impossible, or magic, but that is seems like hair algae blooms are caused by one or a few things being out of whack, and you have to try to figure out what it is.
In my case, I tried the following:
-A variety of grazers (small effect, most prefer anything over hair algae)
-Adding a DI stage to my RO unit (seemed to help a lot, the decline in hair algae started soon afterward)
-Changing from 6500 K to 10000 K lamps (may have helped, decline sped up when they were added)
-Using phosban (may have helped, but algae didn't come back after removing it)
-manual removal (some help, but it will come back fast if conditions are right)
-shortening the MH photoperiod (little effect)
I was running a 10-gallon refugium full of chaetomorpha when the bloom started, so a fuge may help but will probably not cure the problem.
So, like I said, I tried some things and the tank is now Derbesia-free. Because it wasn't horribly scientific, I can't say for sure, but my guess is that the nutrients left by the RO helped feed the algae. It may not have been a problem in the old tank because the light was less intense, then caused a plague when I shifted to very high PAR Iwasaki halides.
So what are your nitrate and phosphate levels right now (and all the other things OG mentioned)?